Preheat the oven to 170°C (338°F).
Melt the dark chocolate and butter together over a double boiler.
In a bowl, beat the eggs and erythritol until frothy.
Add the melted chocolate and butter into the egg mixture and mix well.
Sift the almond flour and cocoa powder into the mixture and combine.
Pour the batter into a mold and level the surface.
Bake in the oven for 20–25 minutes.
Let it cool, then cut into serving sizes.